The present invention relates to an FFT window spread generating method for generating a spread FFT window corresponding to an FFT analysis interval used upon demodulating each received signal modulated in accordance with an OFDM (Orthogonal Frequency Division Multiplex) modulation system by FFT (Fast Fourier Transform).
An OFDM system which transmits a plurality of orthogonal subcarriers (carriers) simultaneously as hitherto, can be used in various applications such as a terrestrial digital television broadcasting system (hereinafter called merely “digital terrestrial broadcasting”) and the like as described in, for example, a patent document (Japanese Unexamined Patent Publication No. Hei 10(1998)-75229).
FIG. 4 is a diagram showing a frame configuration of a transmission signal used in a conventional OFDM system described in the patent document 1 or the like. A demodulating method of the conventional OFDM system will be explained below with reference to FIG. 4.
Each of transmission symbols SBs is constituted of a guard interval (also called “cyclic prefix”) GI and an effective OFDM symbol (hereinafter called simply “effective symbol”) S. The guard interval GI is equivalent to one in which a rear portion Sa of a time waveform of the effective symbol S is extracted and copied onto its head.
In digital transmission using the OFDM system, when distortion and a multipath exist in a transmission line, the orthogonality of a received signal suffers damage or loss and is hence disturbed, so that inter symbol interference (hereinafter called “ISI”) occurs in a demodulated signal, thus causing degradation in error rate. In order to solve this, part of transmission energy (transmission power) is sacrificed, data about a rear portion (corresponding to a period from a few tenth of a whole effective symbol to a fraction thereof) Sa of the effective symbol S desired to be transmitted essentially is used before the effective symbol S, and a guard interval GI for ISI absorption, which is ineffective or invalid as a buffer data portion, is provided. Providing such a guard interval GI enables satisfactory reception without causing ISI if, event though a delayed wave reflected by an obstacle exists in addition to a direct wave, the amount of its delay is shorter than each guard interval GI.
When the transmit signal having such a configuration is sent to the receiving side, the receiving side ignores information of each guard interval GI. Thus, even when a delay occurs only in a given carrier, the delay is ignored if it falls within the guard interval GI, so that the transmit signal can properly be received. Since the data about the rear portion Sa of the effective symbol S is copied onto the corresponding guard interval GI in particular, no information is omitted even though the given carrier is shifted.
Therefore, in the conventional demodulating method, each guard interval GI is removed for every transmission symbol SB from the received OFDM transmission signal by a guard interval removing unit to extract only the effective symbol S. Each extracted effective symbol S is fast discrete Fourier-transformed by an FFT unit, followed by its demodulation.
In the demodulating method of the OFDM system described in the conventional patent document 1 or the like, only the effective symbol S is extracted from the transmission symbol SB with the guard interval GI added thereto and is subjected to fast discrete Fourier-transform, followed by its demodulation. A received signal waveform equivalent to the same length as the guard interval GI is not used. Therefore, power efficiency corresponding to the guard interval GI being transmitted is reduced. However, the related art encountered difficulties in solving the reduction in power efficiency.